A semiconductor laser (referred to as a “SDH-type semiconductor laser” hereinafter) having a SDH (Separated Double Hetero Junction) structure which can be formed by one time of an epitaxial growth process is known as a semiconductor layer having low threshold current Ith, for example, from U.S. Pat. No. 2,990,837.
In this SDH-type semiconductor laser, first, a projecting portion is formed on a substrate having a {100} plane as a main surface to extend in the direction of a {110}A plane. In crystal growth on the main surface of the substrate, a light-emitting portion composed of a laminated structure of compound semiconductor layers is formed on the {100} plane of the projecting portion (referred to as the “protection plane” for convenient sake). The light-emitting portion is composed of a laminated structure in which, for example, a first compound semiconductor layer having a first conductivity type, an active layer, and a second compound semiconductor layer having a second conductivity type are laminated in order. In addition, the light-emitting portion has a triangular sectional shape taken along a virtual plane in a perpendicular direction to the extension direction of the projecting portion, and a side (inclined surface) of the light-emitting portion is composed of a {111}B plane. In a MOCVD method (also referred to as a “MOVPE method”), the {111}B plane is generally known as a non-growth plane except particular crystal growth conditions. Therefore, in the case of the SDH-type semiconductor laser, when the light-emitting portion having the {111}B plane as the side surface is formed, “self-growth termination” is maintained in crystal growth of the light-emitting portion even if MOCVD is continued thereafter.
In the specification, for convenience sake, the following notation of crystal planes is described as (hkl) plane and (hk-l) plane.    (h k l) plane    (h k l) planeIn the specification, for convenience sake, the following notation of crystal directions is described as [hkl] direction and [hk-l] direction.    [h k l] direction    [h k l] direction
On the other hand, a non-growth surface is not present in a portion (referred to as a “recess surface” for convenience sake) of the {100} plane serving as the main surface of the substrate, excluding the projecting portion, when MOCVD is continued, a compound semiconductor layer formed by crystal growth from the recess surface completely cover the light-emitting portion. The compound semiconductor layer formed by crystal growth from the recess surface has a structure in which a current-blocking layer positioning layer, a current-blocking layer, and a buried layer are formed in turn on a second compound semiconductor layer. In this structure, the thickness of the current-blocking layer positioning layer can be controlled so that the current-blocking layer is formed during the formation of the compound semiconductor layer by crystal growth from the recess surface before it completely covers the light-emitting portion (particularly, when the compound semiconductor layer comes near to the side surface the active layer formed in the light-emitting portion), thereby forming a structure in which a current can be injected into only the active layer of the light-emitting portion.
In this way, in the SDH-type semiconductor laser, each of the compound semiconductor layers can be formed on the basis of one time of the crystal growth process. In addition, a material having an energy band gap sufficiently higher than that of the active layer, i.e., a material having a low refractive index, is selected as each of the materials used for the compound semiconductor layers (the first compound semiconductor layer and the second compound semiconductor layer) which hold the active layer therebetween in the vertical direction in the light-emitting portion, and the materials used for the current-blocking layer, the buried layer, and the current-blocking layer positioning layer, which are disposed outside the light-emitting portion, so that the active layer is completely covered with a compound semiconductor layer favorable for light confinement. As a result, the shape of a beam emitted from the semiconductor laser having the side surface of the projecting portion as a light emission surface can be brought near a round shape. Namely, in a far field pattern (FFP), θ//≠θ⊥ can be achieved. Alternatively, the shape of a beam emitted from a semiconductor laser may be required to be elliptic, for example, according to the coupling efficiency with a lens or the like. In such a case, θ// of FFP can be controlled to be small by, for example, employing a so-called flare stripe structure in which the width near the edge of the projecting portion is increased.
In the above-described SDH-type semiconductor laser, an improvement in quality of the current-blocking layer (degree of suppression of a current leakage) is a very important technical factor.